The present invention relates to an apparatus for driving an objective lens in a direction perpendicular to an optical axis of the objective lens.
Such an objective lens driving apparatus may be advantageously applied to an optical scanning apparatus in which a scanning light spot is projected by an objective lens onto information tracks recorded spirally or concentrically on a disc-shaped record medium to write in or read out an information signal along the tracks. In such an apparatus the light spot should follow the track having very narrow width and pitch in a very precise manner, and for this purpose the apparatus is provided with focussing control mechanism and tracking control mechanism. In the focussing control mechanism, the objective lens is driven in a focussing direction parallel with its optical axis so as to make a diameter of the light spot projected on the record medium as small as possible and in the tracking control mechanism, the objective lens is moved in a tracking direction perpendicular to the optical axis and to a track tangential direction.
In U.S. patent application Ser. No. 139,756 assigned to the same assignee as the present application, there has been proposed an objective lens driving apparatus which comprises first and second magnet members arranged symmetrically with respect to a plane which includes an optical axis of an objective lens and is at right angles with respect to a tracking direction perpendicular to the optical axis and to a track tangential direction, first and second magnetic members arranged across the same poles of said first and second magnet members, a third magnetic member arranged within a space surrounded by said first and second magnet members and first and second magnetic members movably in the tracking direction together with the objective lens, and first and second coils wound around said first and second magnetic members, respectively substantially at middle portions thereof, whereby the third magnetic member and objective lens are moved in said tracking direction by supplying a tracking control signal to said first and second coils. In such an objective lens driving apparatus, the third magnetic member and objective lens are supported movably in the tracking direction by means of a resilient supporting member such as a pair of leaf springs extending in the optical axis direction. Then such a supporting member has a resonance point due to a mechanical vibration. At the resonance point, the tracking control signal and the displacement of the objective lens in the tracking direction might be inverted in phase and thus, an accurate tracking control could not be effected. In order to suppress such a resonant vibration, in the above mentioned application magnetic fluids are applied in spaces between the third magnetic member and the coils. In general, a viscosity of the magnetic fluid is largely dependent upon temperature, so that the dumping characteristic is also varied in accordance with temperature change. Moreover, the dumping property is greatly influenced by an amount of the applied magnetic fluids and therefore, it would be rather difficult to manufacture the objective lens driving apparatus having a uniform dumping property on a mass production scale.